This invention relates to improved processes for the isomerization of normal butane to isobutane using a deisobutanizer in combination with a membrane that is selectively permeable to normal butane as compared to isobutane and to improved processes using a membrane to recover at least a portion of isobutane contained in the normal butane-containing feedstock for the isomerization. Processes for the isomerization of normal butane to isobutane are widely practiced. The isomerization process proceeds toward a thermodynamic equilibrium. Hence, the isomerate will still contain a substantial concentration of normal butane, usually in the range of a mole ratio of isobutane to normal butane of 1.2:1 to 2:1. The sought isobutane product, usually having a purity of at least 80, often at least 90 or more, e.g., 95 to 99, mol-% isobutane, is obtained by distillation (deisobutanizer) to obtain the relatively pure isobutane product as an overhead and a normal butane-containing fraction which is recycled to the isomerization reactor.
As the boiling points of normal butane and isobutane are relatively close and a relatively pure isobutane product is desired, the deisobutanizer typically is operated with a high reflux ratio. Thus, the heat duty of the deisobutanizer is a significant component of the operating costs of a butane isomerization process, and the heat duty becomes increasingly significant as higher purity isobutane product streams are sought. Accordingly, improved normal butane isomerization processes are sought that have improved capital and operating costs.
Separation of linear from branched paraffins, e.g., normal butane and isobutane, has been proposed, but membranes have yet to find a practical, commercial application. U.S. Pat. No. 5,069,794 discloses microporous membranes containing crystalline molecular sieve material. At column 8, lines 11 et seq., potential applications of the membranes are disclosed including the separation of linear and branched paraffins. See also, U.S. Pat. No. 6,090,289, disclosing a layered composite containing molecular sieve that could be used as a membrane. Among the potential separations in which the membrane may be used that are disclosed commencing at column 13, line 6, of the patent include the separation of normal paraffins from branched paraffins. U.S. Pat. No. 6,156,950 and U.S. Pat. No. 6,338,791 discuss permeation separation techniques that may have application for the separation of normal paraffins from branched paraffins and describe certain separation schemes in connection with isomerization. US 2003/0196931 A1 discloses a two-stage isomerization process for up-grading hydrocarbon feeds of 4 to 12 carbon atoms. The use of zeolite membranes is suggested as a suitable technique for separating linear molecules. See, for instance, paragraphs 0008 and 0032. U.S. Pat. No. 6,818,333 discloses thin zeolite membranes that are said to have a permeability of n-butane of at least 6·10−7 mol/m2·s·Pa and a selectivity of at least 250 of n-butane to isobutane.
Due to the volumes of normal butane-containing feeds that are processed in commercial-scale butane isomerization units, large membrane surface areas would have to be provided in order to achieve the sought separation of the linear paraffins. For instance, ZSM-5/Silicalite (MFI) membranes (a sieving membrane) available from NGK Insulators, Ltd., Japan, that have selectivity for the permeation of linear paraffins over branched paraffins, have a flux under operating conditions in the range of 0.1 to 1.0 milligram moles per second per square meter at a pressure differential of 15 to 500 kPa. Thus, the costs for commercially implementing such a membrane separation system using these membranes or the membranes of the type disclosed in U.S. Pat. No. 6,818,333 render it not competitive with respect to an adsorption separation system or a distillation separation such as a deisobutanizer.
Recently, Bourney, et al., in WO 2005/049766 disclose a process for producing high octane gasoline using a membrane to remove, inter alia, n-pentane from an isomerized stream derived from the overhead of a deisohexanizer. A side cut from the deisohexanizer is as a sweep fluid on the permeate side of the membrane. The mixture of the permeate and sweep fluid is recycled to the isomerization reactor. In a computer simulation based upon the use of an MFI on alumina membrane, example 1 of the publication indicates that 5000 square meters of membrane surface area is required to remove 95 mass percent of n-pentane from the overhead from a deisohexanizer distillation column. At the flow rate of feed to the permeator (75000 kg/hr. having 20.6 mass percent n-pentane), the flux of n-pentane used in the simulation appears to be in the order of 0.01 gram moles/m2·s at 300° C.
For the purposes of the following discussion of the invention, the following membrane properties are defined.    Microporous
Microporous and microporosity refer to pores having effective diameters of between 0.3 to 2 nanometers.    Mesoporous
Mesoporous and mesoporosity refer to pores having effective diameters of between 2 and 50 nanometers.    Macroporous
Macroporous and macroporosity refer to pores having effective diameters of greater than 50 nanometers.    Nanoparticle
Nanoparticles are particles having a major dimension up to 100 nanometers.    Molecular Sieves
Molecular sieves are materials having microporosity and may be amorphous, partially amorphous or crystalline and may be zeolitic, polymeric, metal, ceramic or carbon.    Sieving Membrane
Sieving membrane is a composite membrane containing a continuous or discontinuous selective separation medium containing molecular sieve barrier. A barrier is the structure that exists to selectively block fluid flow in the membrane. In a continuous sieving membrane, the molecular sieve itself forms a continuous layer that is sought to be defect-free. The continuous barrier may contain other materials such as would be the case with mixed matrix membranes. A discontinuous sieving membrane is a discontinuous assembly of molecular sieve barrier in which spaces, or voids, exist between particles or regions of molecular sieve. These spaces or voids may contain or be filled with other solid material. The particles or regions of molecular sieve are the barrier. The separation effected by sieving membranes may be on steric properties of the components to be separated. Other factors may also affect permeation. One is the sorptivity or lack thereof by a component and the material of the molecular sieve. Another is the interaction of components to be separated in the microporous structure of the molecular sieve. For instance, for some zeolitic molecular sieves, the presence of a molecule, say, n-hexane, in a pore, may hinder 2-methylpentane from entering that pore more than another n-hexane molecule. Hence, zeolites that would not appear to offer much selectivity for the separation of normal and branched paraffins solely from the standpoint of molecular size, may in practice provide greater selectivities of separation.
C4 Permeate Flow Index
The permeability of a sieve membrane, i.e., the rate that a given component passes through a given thickness of the membrane, often varies with changes in conditions such as temperature and pressure, absolute and differential. Thus, for instance, a different permeation rate may be determined where the absolute pressure on the permeate side is 1000 kPa rather than that where that pressure is 5000 kPa, all other parameters, including pressure differential, being constant. Accordingly, a C4 Permeate Flow Index is used herein for describing sieve membranes. The C4 Permeate Flow Index for a given membrane is determined by measuring the rate (gram moles per second) at which a substantially pure normal butane (preferably at least 95 mass-% normal butane) permeates the membrane at approximately 150° C. at a retentate side pressure of 1000 kPa absolute and a permeate-side pressure of 100 kPa absolute. The C4 Permeate Flow Index reflects the permeation rate per square meter of retentate-side surface area but is not normalized to membrane thickness. Hence, the C4 Permeate Flow Index for a given membrane will be in the units of gram moles of normal butane permeating per second per square meter of retentate-side membrane surface area.
C4 Permeate Flow Ratio
The C4 Permeate Flow Ratio for a given sieve membrane is the ratio of the C4 Permeate Flow Index (n-butane) to an i-C4 Permeate Flow Index wherein the i-C4 Permeate Flow Index is determined in the same manner as the C4 Permeate Flow Index but using substantially pure isobutane (preferably at least 95 mass-% isobutane).